Historically, bonding materials quickly without use of mechanical fasteners has been quite difficult. Some of the problem areas which exist include: assembly time, cost of materials and labor, quality of the bond between component structures being assembled, reliability of the process of assembly, the typical requirement of fairly difficult finishing steps, convenience (or lack thereof) of use for the end-users, worker safety issues, and the difficulty in maintaining a good quality of appearance of the finished goods.
In the 1940's, mechanical fasteners dominated the assembly industry, and adhesives were not as important to industry during this period. From the 1930's through World War II, the United States and Germany began to develop plastics and adhesives technology in response to the growing scarcity of natural products. In many cases, particularly early on, adhesives have been used either in combination with mechanical fasteners or where no mechanical fastener could be effectively employed. Beginning in the 1950's, the modern adhesives industry began to develop. Some of today's more common adhesive systems, developed at that time, included heat-curable thermosets (epoxies), thermoplastic hot melts, pressure-sensitive adhesives (PSA's), contact cements, water-based wood glues, and the super glues (cyanoacrylates). These were major disruptive technologies that have evolved over the last 45 years and which have slowly grown the fastening market and have significantly replaced traditional mechanical fasteners in many markets.
Adhesive bonding is generally superior to mechanical fastening, but present technology doesn't allow for cost-effective pre-positioning and rapid development of a strong bond on demand with one step. Pre-positioning of components, prior to fastening, is very important, particularly in non-automated assembly systems. Millwork is an excellent example of such an assembly system. No current adhesive system allows for pre-positioning coupled with instantaneous bonding. Most of today's adhesives are slow to cure, requiring minutes to hours, thus requiring clamping and other direct personal attention by the installer.
It should be noted that the ideal adhesive system is one where the adhesive cures on demand, is reversible on demand, has unlimited shelf life, has no VOC's (volatile organic compounds), and is safe and easy to handle. Currently, the only such systems that exist are the light-curable systems, such as those employing UV (ultraviolet) and visible light. UV and visible light systems are unique in today's adhesives world. They are liquid systems that cure only upon exposure to light. Optically transparent parts can be bonded in seconds or less to virtually any substrate. Such systems, where useful, have virtually replaced all other adhesive or mechanical fastening systems. An example would be automotive headlamp assemblies that do not need to be disassembled. It should be noted that UV-curable adhesives involve chemical handling and are not currently reversible.
There are two basic types of adhesive systems: one group of systems allows for pre-positioning of the parts to be bonded, yet by default, requires long cure times; the other group of systems provides very short, almost instantaneous cure times, but yet prevents pre-positioning of the parts.
Before describing some of the major adhesive systems available, one should be aware of the following general application notes that affect adhesive utility.
(1) Many product assembly sites are often dirty and difficult to keep clean. Certain adhesive systems cannot handle such situations.
(2) Temperature fluctuations at many assembly sites could be extreme, whether for an interior or exterior application. For example, a new home being built in the middle of the winter could see interior temperatures below 0° F. Exterior applications could easily see temperatures even lower. Another typical example could involve automotive body repair, if done inside a non-heated building.
(3) Where humidity may be important, it is clear that the humidity around a manufacturing facility in Arizona would be far lower than that in and around a facility in Florida.
(4) The ability to directly heat many product components to cure adhesives is extremely limited, particularly as many plastic components can melt, and wood-based or cellulose-based millwork can burn.
One family of fast-curing adhesives is called “super glues” (cyanoacrylates). These adhesives allow for an extremely rapid adhesive setting, but cannot in any way be pre-positioned before placement as the adhesive cures during positioning. Thus, there is no room for error. These adhesives are generally the most expensive adhesives. Furthermore, they are difficult to handle, and have a limited shelf life. Finally, there is no way to easily reverse cyanoacrylate, or super glue, bonds. Companies in this industry include Loctite Corporation, Henkel A.G., and National Starch.
Another instant adhesive technology, not often employed in structural applications, is pressure-sensitive adhesive (PSA) tapes. Like super glues, such products allow for extremely rapid adhesive bonding, but again, are extremely limited with regard to pre-positioning and, as with the “superglues,” again, there is no room for error. Furthermore, pressure-sensitive adhesives are limited in their ultimate strengths unless they are thermosetting. In the case of a thermosetting PSA, some form of heat- or moisture-activation is required which is generally impractical for non-heat-resistant products, or where humidity controls are unavailable.
The latter two above thermosetting processes are time intensive. Even more importantly, pressure sensitive adhesives can be applied only in very narrow temperature ranges, typically from 55° F. to 90° F. Furthermore, above 90° F., many common PSA's weaken dramatically. As a further note on PSA's, they are incapable of flow without heat to accommodate uneven surfaces, and if exposed to dust or other particulates, they immediately lose much of their potential adhesive strength. Finally, it is extremely difficult, if not impossible in most cases, to disassemble parts that use PSA's. Examples of companies that manufacture PSA's are 3M and Avery-Dennison, which are the two largest of the group. The cost of PSA's can range from being some of the most inexpensive to some of the most expensive adhesives available today.
Hot melt adhesives are another example of an instantaneous or fast-cure system that significantly limits the ability to pre-position parts. Such adhesives are melted either in a large tank or in a small glue gun and are then dispensed as a molten material onto the parts. The parts are then quickly mated, and the bond forms as the adhesive cools. The cooling process can be as short as a few seconds to possibly as long as ten or twenty seconds. As with the other instantaneous adhesives, there is little room for error, particularly where a clean and thin bond line is desired. Such limitations are the reasons that hot melt adhesives are used most extensively in the packaging industry and also for bonding small parts or surface areas. They are particularly useful in highly automated production systems, such as for sealing cereal boxes. Furthermore, such adhesives cannot be reheated after product assembly without significantly or entirely heating the product assembly.
On the positive side, hot melt adhesives are one-component, solid-state, zero VOC systems that have indefinite shelf life and, for the most part, are considered as plastics for regulatory and safety purposes. Furthermore, most hot melt adhesives are moderate to low in cost, especially when compared to the super glues or the light-curable adhesives. Examples of some leading hot melt manufacturers are Henkel A.G., Jowat, National Starch, H.B. Fuller, and Ato-Findley.
Other types of adhesive systems are those which are pre-positionable, but have long cure times. The most well known pre-positionable adhesives are the epoxies. Epoxy adhesives generally have slow cure times, usually on the order of minutes to hours, or even days. Most epoxies are two-part systems that, when mixed, become activated and cure. The catalysts are in one or both parts and their concentrations determine how quickly the epoxy adhesive will cure. In fact, if enough catalyst is added, epoxies can become instantaneous systems that are not pre-positionable. Epoxies are not difficult to handle, but do require special care as exposure can sometimes be detrimental to human health (causing skin irritations and burning).
Epoxies are among the strongest adhesives known, but require heat to achieve ultimate strength. A major problem with two-part epoxies is that cure time can vary dramatically with temperature. In fact, some systems cure so rapidly at temperatures above 90° F. that they become almost unusable. At colder temperatures, e.g., below 60° F., some systems may take days or more to cure. There also are one-component epoxies that cure only upon exposure to heat. Once heated, many one-component systems can cure in less than one minute. Epoxy bonds cannot be easily reversed. Examples of leading epoxy manufacturers include Ciba-Giegy, Shell Chemical, Henkel A.G., and Loctite.
Urethanes are another well-known, pre-positionable adhesive group. Like the epoxies, there are both two-part and one-part systems. After epoxies, urethanes are probably the second strongest class of commonly used adhesives. Two-part systems are the most common and generally take minutes to hours to cure. There are many one-part systems becoming available today which are moisture-curable (the moisture is actually a second part). Both systems have the problem that one component of the two, the isocyanate, is moisture-sensitive. If water gets into the adhesive, or if the humidity is too high, the isocyanate will react with the water, generate a gas, and cause foaming to occur. Even worse, if the moisture gets into a container unbeknownst to the user, and the container is then closed, the container can explode. As a result, two-component and moisture-cure urethanes are generally only used by skilled or specially trained personnel. Furthermore, because of their reactive nature and environmental susceptibility, most urethane adhesive systems require specialized mixing and dispensing equipment that must be cleaned on a frequent basis.
The primary advantage of most urethane adhesives is the availability of room temperature, moisture-curing, one-part systems that possess an overall lower application viscosity. This is as opposed to a two-part, room temperature epoxy that must be mixed, or a one-component hot melt that must be melted. Applications for urethane adhesives range from automotive assembly, to marine and aerospace assembly, to the millwork, furniture, and cabinetry industries.
It is important to note that certain adhesives have already been used with induction devices for many years. For example, such technologies are used for high strength bonds using relatively long cure-time (fifteen minutes to hours) adhesives. Furthermore, this technology generally employs high pressures to facilitate bond formation. This technology is used, for example, by Boeing, in the construction of composite-based passenger aircraft. The adhesive systems employed by Boeing are mainly epoxies. Such adhesives must be pre-positionable, and further must be cured over a long period of time because of the strict performance requirements mandated by the government for passenger and military aircraft.
Another company that employs similar technology is Emabond, a subsidiary of Ashland Chemical. Emabond develops the same types of long-time-cure adhesives (epoxies) as does Boeing, however, Emabond employs particulate susceptors which activate at higher frequencies that require operator shielding for safety. Emabond equipment is primarily geared toward automotive component assembly. A special piece of induction equipment is typically required for any two automotive components to be assembled.
Emabond employs a number of adhesive technologies, including epoxies, urethanes, and hot melt adhesives. Most of the adhesive systems used by Emabond are heat-activated by particulate susceptors, not foil susceptors, at higher frequencies that are known to be dangerous to human health (e.g., above 5 MHz). Moreover, the Emabond systems, primarily for the automotive industry, are part specific and are designed to bond generally irregular surfaces. The particulate susceptors allow for the use of liquid adhesives that can easily conform to these irregularities.
One method of bonding structures together utilizes susceptors made of an electrically conductive material that is heating by an alternating magnetic field to activate an adhesive material that resides on at least one surface of the susceptor. The magnetic field induces electrical currents, known as eddy currents, in the electrically conductive media. Exposure of such electrically conductive media to a magnetic field causes a temperature rise (heating) by what is termed the Joule effect. The Joule effect relates to heat generation due to the flow of electrons in a conductor. Distributions of these electrical currents and the heat they produce are not uniform in a conductive medium, such as a susceptor, exposed to an alternating magnetic field. The magnitude of heat, in Watts, is the sum of the heat contributions of all eddy current paths within the susceptor, each of which contributes heat that is equivalent to the product of its electrical resistance in Ohms and the square of its electron current in Amperes.
Within non-ferromagnetic susceptors, induced eddy currents have maximum intensities at the surfaces nearest the incident alternating magnetic field and have reduced intensities within the material, decreasing exponentially as a function of depth. This phenomenon is known as the skin effect, or the Kelvin effect, and the depth at which the eddy current falls to 37% is known as the depth of penetration. Most susceptors employed in the present application are comprised of a thin conductive sheet of uniform (or purposely nonuniform thickness) where, for the low frequencies usually used, the depth of penetration is far greater than the material thickness. Eddy currents at all depths within these susceptors are thus approximately equal, except where purposeful variations in susceptor thickness, or where open-space across the width of the susceptors cause variations in current density. In such cases, currents are forced to be non-uniform in specific regions to create more uniform heat generation or less uniform heat generation, depending on the specific application.
The magnitude of heat generated within a susceptor comprised of a conductive sheet of uniform thickness, is related to several factors. These factors include susceptor permeability, resistivity, size and shape, and the magnitude, frequency, size, and shape of the incident AC magnetic field. Variations of many of these parameters interrelate and affect the current distributions and densities that affect the sizes and locations of useful heat sources within the susceptors.
A Canadian patent by Krzeszowski, C A 1,110,961, (which is similar to U.S. Pat. No. 4,123,305) discloses a method for inductively heating a thermo-fusible material interposed between a carpet and a floor. An inductive heating tool is used to raise the temperature of a relatively thin-foil susceptor, which in turn activates the thermo-fusible adhesive material to create a bond, and thus “glue” the carpet to the floor. Krzeszowski discloses the use of a sheet of the thermo-fusible adhesive material, which is first placed upon the floor, followed by the carpet. Krzeszowski discloses the use of both continuously perforated sheets of aluminum as the susceptor material, or solid aluminum sheet. In one embodiment, a “vapour-barrier” sheet of aluminum (i.e., without perforations) is glued onto a slab of plaster, and then its other side is glued to a slab of expanded polystyrene, thereby creating a moisture barrier panel. One preferred aluminum sheet material disclosed in Krzeszowski is “ALBAL brand, reference 623,” either with or without perforations.
The Boeing Aircraft Company owns several patents in the field of inductively heated susceptors. Virtually each patent extols the value of “even heating” of the susceptor to form a very high-strength and uniform bond. Of course, for aircraft structures, high strength bonds can be critical. Such patents include U.S. Pat. No. 3,996,402 (by Sindt), U.S. Pat. No. 5,717,191 (by Cristensen), U.S. Pat. No. 5,916,469 (by Scoles), and U.S. Pat. No. 5,500,511 (by Hansen). These patents use susceptors having various openings, and in some cases the openings are so large and numerous that the susceptor has an appearance of a screen-like material. All of the susceptors specified by the above Boeing patents have thickness dimensions that exceed 0.003 inches (3 mils). Such devices are not particularly useful in “quick” bonding of substrates.
Previous induction heating devices suffer from an inability to be made truly portable, i.e., lightweight, while simultaneously delivering the energy necessary to form bonds in short periods of time. It would be desirable, especially for higher-speed, lower-strength bonding applications, to provide an induction adhesive activation device with corresponding susceptor design that accumulates the heat in the susceptor and the adhesive while simultaneously withholding significant conduction losses to the substrates until all of the adhesive had either melted, begun chemical reaction, flowed adequately, or all three occurred.
Such a system would be valuable if the bonds developed were as strong as typically required for as wide a range of applications as possible, and it would be even more valuable if the susceptor adhesive device were optionally reversible by design. Such an induction adhesive activation device would ideally have improved energy efficiencies, sufficient to enable operation with a battery, be lightweight, support high duty-cycle operation (>40%) for many hours at a time, and require no liquid cooling.
It would be advantageous to provide an induction-based adhesive technology that can bond nearly instantaneously on demand, and which is not directed toward a pre-positionable adhesive, thereby allowing for simplified, more rapid production, and eliminating the requirement of high-energy systems such as those that operate at high frequencies that are known to be dangerous to human health.